Cinusa nippon n. sp. is described as the second species of the genus from ovigerous females and adult males found in the buccal cavity of three species of coastal puffers [vermiculated puffer, Takifugu snyderi (Abe, 1989) (type host); panther puffer, Takifugu pardalis (Temminck and Schlegel, 1850); and fine patterned puffer, Takifugu flavipterus ], in the southern Sea of Japan off western Honshu, Japan. The female of the new species is distinguished from that of its single congener, Cinusa tetrodontis , by the shape of pereonite 1 (straight lateral margins with slightly produced anterolateral angles in the new species vs. rounded anterolateral sides and angles in C. tetrodontis). The number of recurved robust setae on the female's maxilliped article 3 is also different between the two species: C. nippon n. sp. has two robust setae, while C. tetrodontis has three robust setae. Moreover, the male of the new species is differentiated from that of C. tetrodontis in the number of antennal articles and recurved robust setae on the maxilla and maxilliped. Cinusa nippon n. sp. represents the first specimen-based record of the genus from the Pacific region. An amended diagnosis of the genus is provided.